(1) Field of the Invention
The present invention relates to a cladding material for an optical fiber. More particularly, the present invention relates to a cladding material for an optical fiber, which has an excellent adhesion to a core of an optical fiber, bending resistance and transparency. A core-clad optical fiber having this cladding material is characterized in that, even when the optical fiber is bent, a satisfactory light-transimitting property can be retained.
(2) Description of the Related Art
It is known that a core-clad optical fiber is valuable as an optical fiber, and known core-clad optical fibers are divided into three types, i.e., an optical fiber in which both ot the sheath and core are composed of plastics, an optical fiber in which the core is composed of glass and the sheath is composed of plastics, and an optical fiber in which both the core and sheath are composed of glass. Of these core-clad optical fibers, optical fibers having a sheath composed of plastics have a good flexibility and a relatively high bending resistance, and therefore, these optical fibers are easier to handle than optical fibers composed entirely of glass and are advantageously used as an intermediate-range communication medium or a light-transmitting medium of a display comprising an optical fiber.
As the clad-forming polymer for an optical fiber, a fluoroalkyl methacrylate polymer is disclosed in Japanese Examined Patent Publication No. 43-8,978, a trifluoromethyl methacrylate polymer is disclosed in Japanese Unexamined Patent Publication No. 49-107,790, and a vinylidene fluoride/tetrafluooethylene copolymer is disclosed in Japanese Examined Patent Publication No. 53-21,660.
Of these clad-forming polymers, the vinylidene fluoride/tetrafluoroethylene copolymer has a low refractive index and has excellent mechanical characteristics and adhesion to the core component. However, since this copolymer is crystalline to some extent, the copolymer has a poor transparency, and therefore, a core-clad optical fiber comprising a clad composed of this vinylidene fluoride/tetrafluoroethylene copolymer does not have the required light-transmitting characteristics.
The fluoroalkyl methacrylate polymer has a good transparency and a low refractive index, but the light-transmitting property of a core-clad optical fiber comprising a clad composed of this polymer is easily lost by bending.
Accordingly, investigations have been made into the development of cladding materials not having the above-mentioned defects. For example, Japanese Unexamined Patent Publication No. 61-66,706 discloses, as the cladding polymer, a copolymer comprising 30 to 60% by weight of a long-chain fluoroalkyl methacrylate represented by the following formula: ##STR3## 30 to 60% by weight of a short-chain methacrylate represented by the following formula: ##STR4## wherein X is H or F, and n is an integer of from 1 to 4, 20 to 50% by weight of a short-chain fluoroalkyl methacrylate represented by the following formula: ##STR5## wherein X is H or F and n is an integer of from 1 to 4, and 20 to 50% by weight of methyl methacrylate
The clad composed of this copolymer has a higher adhesion to the core of the optical fiber and a higher bending resistance than the conventional clad of the perfluoroalkyl methacrylate copolymer, and the optical fiber having this clad showed a high bending resistance of about 15 to 16 bending at the 180.degree. bending test.
In this optical fiber, however, the light-transmitting property is drastically degraded in the bent state. For example, supposing that the light-transmittance of this optical fiber is evaluated as 100 in the normal state, the light-transmitting property is reduced substantially to zero and light is not transmitted at all in the state where the optical fiber is wound by 100 turns on a round rod having a diameter of 10 mm. If the optical fiber is unwound from the round rod and straightened and the light-transmitting property is measured again, the light-transmitting property is then as low as 40 to 60.
Conventional optical fibers having a plastic core have a higher flexibility than glass type optical fibers and it is considered that these optical fibers have a good processability and are ease to handle. But, as pointed out above, the light-transmitting property is drastically degraded in the bent state and the light-transmitting property is very low when the optical fibers are released from the bent state, and thus these optical fibers are not suitable for use as communication media. Therefore, the developement of optical fibers in which these points have been improved is strongly desired.